package com.leetcodehot.problems;

public class problems143 {
    public class ListNode {
        int val;
        ListNode next;

        ListNode() {
        }

        ListNode(int val) {
            this.val = val;
        }

        ListNode(int val, ListNode next) {
            this.val = val;
            this.next = next;
        }
    }
    public void reorderList(ListNode head) {
        ListNode head2 = returnMid(head);
        head2 = reverserListNode(head2);
        while(head2.next != null) {
            ListNode nxt = head.next;
            ListNode nxt2 = head2.next;
            head.next = head2;
            head2.next = nxt;
            head = nxt;
            head2 = nxt2;
        }
    }

    private ListNode returnMid(ListNode head) {
        ListNode slow = head;
        ListNode fast = head;
        while (fast != null && fast.next != null) {
            slow = slow.next;
            fast = fast.next.next;
        }
        return slow;
    }

    private ListNode reverserListNode(ListNode head) {
        ListNode pre = null;
        ListNode cur = head;
        while (cur != null) {
            ListNode nxt = cur.next;
            cur.next = pre;
            pre = cur;
            cur = nxt;
        }
        return pre;
    }
}
